"there can be only one" is a common saying from the Highlander series of movies and TV shows, which refers to an in-universe event where "immortals" (they can be killed by decapitation) feel compelled to "the gathering" and slay each other until the last one receives "the gift".

The series is named "Highlander" because the protagonist in the first movie, Connor MacLeod, is an immortal from the highlands of Scotland and often called "the highlander" by immortals, since he may be living under a different name (because even in fiction paperwork is a bitch).

Outside of the series, "highlander" variants of various games exist which impose a limit of a single instance of a card/token/spell/action where the standard rules have a higher limit or no limit.